Known color diffusion transfer film units are divided into a peel-apart type and a monosheet type (non-peel type). The peel-apart type film unit is composed of a light-sensitive element and a dye-receiving element on separate supports. After imagewise exposure, the light-sensitive element and the image-receiving element are brought into contact, a processing solution is spread therebetween, and the dye image-receiving element is peeled off to obtain a dye image transferred on the dye image-receiving layer.
The monosheet type film units comprise a pair of substrates one of which is transparent, between which a dye image-receiving element, a light-sensitive element, and a neutralization timing element are provided. The light-sensitive element may be provided on the same transparent support on which the dye-image receiving element is provided, or may be provided on the separate support. In the former case, a white reflecting layer is provided between the image-receiving element and the light-sensitive element, and in the latter case a processing solution to be spread between the image-receiving element and the light-sensitive element contains a white pigment, so that the dye image transferred to the image-receiving layer may be seen by reflected light.
A color diffusion transfer system using an alkali processing composition as one of the elements constituting the film unit has a disadvantage that a highly sharp image is hardly obtained; because the distance of diffusion of a dye generated is long, the system involves fixing of the dye, and the black or white pigment used in a light-shielding layer or in a white background layer acts as resistance against diffusion.
In such an image formation system including diffusion and fixing of a dye, it is important to minimize the distance of diffusion of the formed dye, that is, the thickness of the film unit, in order to obtain a clear image in a short time. In order to achieve this, it is effective to reduce the amount of a binder that is resistance against dye diffusion and/or to develop a photographically useful substance that would function at a low amount of addition. Development of a technique enabling reduction of film thickness has been a subject for not only a color diffusion transfer system but other photographic systems.
Pigments usually used in color diffusion transfer film units, such as carbon black and titanium white, are very apt to agglomerate, causing such fatal problems as a leak of light or white background stains. To avoid these problems, it has been a practice to use these pigments or a binder in amounts more than necessary, which has made it difficult to reduce the film thickness. Any means other than thickness reduction taken to accelerate image formation would deteriorate the image sharpness.
It has therefore been demanded to develop a technique for reduction in film thickness or a technique for increasing the rate of dye diffusion to obtain a clear image in a reduced time.
It has been proposed to incorporate a tertiary amine polymer latex into a color diffusion transfer film unit as a photographically useful substance for improving sharpness and the like. After a transfer image is formed, and the pH of the system decreases, the amine moiety of the tertiary amine polymer latex becomes capable of capturing the residual dye which lags behind in the transfer, considerably suppressing an unnecessary increase in image density after image formation. The tertiary amine polymer having such an action can be used in any of light-sensitive layers and a mordanted layer. However, the tertiary amine polymer latex has poor stability and easily coagulates to reduce filterability. Therefore, there has been a demand to develop a technique to improve the stability of the tertiary amine polymer latex.